1. Field of the Invention
The present invention relates to an organic light emitting diode, and more particularly to a pixel structure using an organic light emitting diode which can prevent characteristic deterioration of driving transistors for driving a voltage programming type active matrix organic light emitting diode due to voltages being applied to the driving transistors.
2. Description of the Related Art
Recently, thin light inexpensive display devices having high efficiency have been actively developed, and one of such remarkable next-generation display devices is an organic light emitting display device. This organic light emitting display device uses an elector-luminescence (EL) phenomenon of specified organic compounds or high polymers, and thus it is not required to adopt a backlight in a display device. The display device using the EL phenomenon can be thinner than a general LCD, can be inexpensively and easily manufactured, and has the advantages of a wide viewing angle and bright light.
An organic light emitting display device using organic light emitting diodes (OLEDs) is provided with OLEDs and thin film transistors (TFTs) for driving the OLEDs. This TFT is classified into a poly silicon TFT, an amorphous silicon TFT, and others, depending on the kind of its active layer. Also, the type of the TFT is classified into an active-matrix type and a passive-matrix type, depending on the existence/nonexistence of switching elements provided in a unit pixel of an organic light emitting display panel.
Although the organic light emitting display device adopting the poly silicon TFTs has various kinds of advantages and thus has been generally used, the TFT manufacturing process is complicated with its manufacturing cost increased. In addition, it is difficult to achieve a wide screen in the organic light emitting display device adopting the poly silicon TFTs. By contrast, it is easy to achieve a wide screen in the organic light emitting display device adopting the amorphous silicon TFTs, and this organic light emitting display device can be manufactured through the relatively small number of manufacturing processes in comparison to the organic light emitting display device adopting the poly silicon TFTs. However, as the amorphous silicon TFTs continuously supply current to the OLED, the threshold voltage VTH of the amorphous silicon TFT itself may be shifted so as to cause the amorphous silicon TFT to deteriorate. Also, due to this, non-uniform current may flow through the OLED even if the same data voltage is applied thereto, and this causes the deterioration of picture quality of the organic light emitting display device to occur.
FIG. 1 is a circuit diagram of a unit pixel of a conventional voltage programming type active matrix OLED. This voltage programming type active matrix OLED is composed of two TFTs and one capacitor. In FIG. 1, the first TFT T1 serves as a switch such as an active matrix LCD, the capacitor CSTG stores a data voltage, and the second TFT T2 serves to flow current corresponding to the value of the data voltage stored in the capacitor CSTG to the OLED.
However, the voltage programming type basic pixel structure as illustrated in FIG. 1 has a drawback in that if the threshold voltage of the second TFT T2 deteriorates due to a continuous supply of a gate bias voltage, deteriorating voltage flows to the OLED through the second TFT T2 although the same data voltage is charged in the capacitor CSTG. Accordingly, the pixel structure as illustrated in FIG. 1 cannot correct the deterioration of the TFT threshold voltage between pixels at all. The current flowing between the source and drain of the second TFT T2 appears as the following current-voltage relational expression in a saturation region.ID=½×k×(VGS−VTH)2 
Here, k=μ×Cins×W/L, and μ denotes a field effect mobility, Cins denotes the capacitance of an insulating layer, W denotes the channel width of a TFT, and L denotes the channel length of the TFT.